Since physicochemical conditions were most probably
extreme on the primitive Earth, one can imagine that actual
extremophiles are the progeny of the first inhabitants of
Earth. This hypothesis is particularly attractive, looking at
the universal phylogenetic tree of life based on 16S rRNA
genes. For that tree, the most ancient branches for Bacteria
and Archaea correspond to hyperthermophilic organisms.
However, this hypothesis received several criticisms, but
one must say that the question of temperature (high, moderate, low) that allowed the origin of life is still unanswered.
In addition to the extreme values of physiochemical
parameters to which extremophiles are exposed nowadays,
the metabolism of extremophiles has a great interest. Some
of them utilize organic carbon as carbon and energy sources
through various processes of fermentation. But many
extremophiles are lithotrophic (inorganic energy source),
anaerobic (their terminal electron acceptor is not molecular
oxygen), and autotrophic, using carbon dioxide as sole carbon source. Actually, many extremophiles and mostly
hyperthermophiles thrive in environments where nutritional
conditions may resemble to those expected on primitive
Earth. Some extremophiles that live in subsurface
environments seem to be independent from solar energy
and compounds produced by photosynthesis: it is what
occurred more than 2.5 billion years ago. In this respect,
the study of actual extremophiles may considerably help us
to understand our far origins. But, for sure, the whole planet
and its biological diversity are not totally explored yet.
Several centuries ago, the Italian Dominican monk
Giordano Bruno was burned after he claimed that many
planets moved around their sun, such as our 7 (right number
at that time) planets around the sun, and many organisms
lived there. While in 2010 NASA’s Phoenix probe explored
Mars, the European ExoMars probe is preparing to drill the
red planet, and a sophisticated satellite should go to the moons
of Jupiter in less than 10 years to finally prove the existence of
an ocean of liquid water beneath the ice crust of Europa.
Clearly, there was liquid water on Mars when life appeared
on Earth. What happened then on the red planet? Did life
start? Did life still exist, perhaps in the subsurface? Or, are
there some fossils or signatures somewhere? In a further step,
after automatic space vessels that carried out in situ analysis,
one may expect that other vessels will collect and bring to
Earth Martian samples, for analysis after severe quarantine.
Later on, manned vehicles will land on Mars. Dreams?
Europa, a natural satellite of Jupiter, is an interesting
target since liquid water is suspected beneath its thick ice
cover, probably because a tide effect from Jupiter. At the
bottom of this 100-km deep ocean, a silicate core should
exist. Consequently, water and rocks are in contact and a
hydrothermal circulation could exist. In the case of Europa,
exobiologists will wait for a long time to get answers. After
one or several orbiter missions, a lander mission is required,
then a drilling mission (at least a few meters, but the ice
cover to drill to reach the ocean is several kilometers thick).
And a journey to Jupiter takes about 5–7 years!
Hopefully, on Earth, analogues to Europa ocean exist: the
subglacial lakes on Antarctica, such as lake Vostok. Deep
drilling through Vostok ice revealed molecular signatures
of microorganisms, some being thermophiles that could
indicate a hydrothermal activity at depth.
If we move out of the solar system, more than 100
extrasolar planets (moving around remote stars) have been
discovered. Some recently observed have masses equivalent
to several Earths. In the next future, planets with the size of
Earth, located in the habitable zone from their star (liquid
water, acceptable temperatures, etc.), are expected.
For all these targets, exobiologists, planetologists, and
astrophysicists search for traces, marks, and signatures for
life. The outstanding properties and capabilities of terrestrial
extremophiles have pushed further the physiochemical, metabolic, and geographical limits for life. Consequently, plausible habitats are more and more numerous. Technologies
required for exploration are more and more powerful. Will
the twenty-first century bring any answer?
References
Aguilera A, Gonzalez-Toril E, Souza-Egipsy V, Amaral-Zettler L, Zettler
E, Amils R (2010) Phototrophic biolfilms from Rio Tinto, an extreme
acidic environments, the prokaryotic component. In: Seckbach J, Oren
A (eds) Microbial mats. Modern and ancient microorganisms in
stratified systems. Springer, Heidelberg, pp 471–481
Alazard D, Joseph M, Battaglia-Brunet F, Cayol JL, Ollivier B (2010)
Desulfosporosinus acidiphilus sp. nov.: a moderately acidophilic
sulfate-reducing bacterium isolated from acid mining drainage
sediments. New taxa: Firmicutes (Class Clostridia, Order
Clostridiales, Family Peptococcaceae). Extremophiles 14:305–312
Amaral-Zettler LA, Go ´mez F, Zettler E, Keenan BG, Amils R, Sogin
ML (2003) Eukaryotic diversity in Spain’s River of Fire. Nature
417:137
Basen M, Sun J, Adams M (2012) Engineering a hyperthermophilic
archaeon for temperature-dependent product formation. mBio 3:
e00053-12. doi:10.1128/mBio.00053-12
Ben Hania W, Ghodbane R, Postec A, Brochier-Armanet C, Hamdi M,
Fardeau M, Ollivier B (2011) Cultivation of the first mesophilic
representative (“mesotoga”) within the order Thermotogales. Syst
Appl Microbiol 34:581–585
Benz M, Brune A, Schink B (1998) Anaerobic and aerobic oxidation of
ferrous iron at neutral pH by chemoheterotrophic nitrate-reducing
bacteria. Arch Microbiol 169:159–165
Biddle J, Cardman Z, Mendlovitz H, Albert D, Lloyd K, Boetius A,
Teske A (2012) Anaerobic oxidation of methane at different temperature regimes in Guaymas Basin hydrothermal sediments. ISME
J 6:1018–1031
Borin S et al (2008) Sulfur cycling and methanogenesis primarily
drives microbial colonization of the highly sulfidic Urania deep
hypersaline basin in the Mediterranean Sea. Proc Natl Acad Sci
USA 106:9151–9156
Brochier-Armanet C, Boussau B, Gribaldo S, Forterre P (2008)
Mesophilic Crenarchaeota: proposal for a third archaeal phylum,
the Thaumarchaeota. Nat Rev Microbiol 6:245–252
392
J.-L. Cayol et al.
extreme on the primitive Earth, one can imagine that actual
extremophiles are the progeny of the first inhabitants of
Earth. This hypothesis is particularly attractive, looking at
the universal phylogenetic tree of life based on 16S rRNA
genes. For that tree, the most ancient branches for Bacteria
and Archaea correspond to hyperthermophilic organisms.
However, this hypothesis received several criticisms, but
one must say that the question of temperature (high, moderate, low) that allowed the origin of life is still unanswered.
In addition to the extreme values of physiochemical
parameters to which extremophiles are exposed nowadays,
the metabolism of extremophiles has a great interest. Some
of them utilize organic carbon as carbon and energy sources
through various processes of fermentation. But many
extremophiles are lithotrophic (inorganic energy source),
anaerobic (their terminal electron acceptor is not molecular
oxygen), and autotrophic, using carbon dioxide as sole carbon source. Actually, many extremophiles and mostly
hyperthermophiles thrive in environments where nutritional
conditions may resemble to those expected on primitive
Earth. Some extremophiles that live in subsurface
environments seem to be independent from solar energy
and compounds produced by photosynthesis: it is what
occurred more than 2.5 billion years ago. In this respect,
the study of actual extremophiles may considerably help us
to understand our far origins. But, for sure, the whole planet
and its biological diversity are not totally explored yet.
Several centuries ago, the Italian Dominican monk
Giordano Bruno was burned after he claimed that many
planets moved around their sun, such as our 7 (right number
at that time) planets around the sun, and many organisms
lived there. While in 2010 NASA’s Phoenix probe explored
Mars, the European ExoMars probe is preparing to drill the
red planet, and a sophisticated satellite should go to the moons
of Jupiter in less than 10 years to finally prove the existence of
an ocean of liquid water beneath the ice crust of Europa.
Clearly, there was liquid water on Mars when life appeared
on Earth. What happened then on the red planet? Did life
start? Did life still exist, perhaps in the subsurface? Or, are
there some fossils or signatures somewhere? In a further step,
after automatic space vessels that carried out in situ analysis,
one may expect that other vessels will collect and bring to
Earth Martian samples, for analysis after severe quarantine.
Later on, manned vehicles will land on Mars. Dreams?
Europa, a natural satellite of Jupiter, is an interesting
target since liquid water is suspected beneath its thick ice
cover, probably because a tide effect from Jupiter. At the
bottom of this 100-km deep ocean, a silicate core should
exist. Consequently, water and rocks are in contact and a
hydrothermal circulation could exist. In the case of Europa,
exobiologists will wait for a long time to get answers. After
one or several orbiter missions, a lander mission is required,
then a drilling mission (at least a few meters, but the ice
cover to drill to reach the ocean is several kilometers thick).
And a journey to Jupiter takes about 5–7 years!
Hopefully, on Earth, analogues to Europa ocean exist: the
subglacial lakes on Antarctica, such as lake Vostok. Deep
drilling through Vostok ice revealed molecular signatures
of microorganisms, some being thermophiles that could
indicate a hydrothermal activity at depth.
If we move out of the solar system, more than 100
extrasolar planets (moving around remote stars) have been
discovered. Some recently observed have masses equivalent
to several Earths. In the next future, planets with the size of
Earth, located in the habitable zone from their star (liquid
water, acceptable temperatures, etc.), are expected.
For all these targets, exobiologists, planetologists, and
astrophysicists search for traces, marks, and signatures for
life. The outstanding properties and capabilities of terrestrial
extremophiles have pushed further the physiochemical, metabolic, and geographical limits for life. Consequently, plausible habitats are more and more numerous. Technologies
required for exploration are more and more powerful. Will
the twenty-first century bring any answer?
References
Aguilera A, Gonzalez-Toril E, Souza-Egipsy V, Amaral-Zettler L, Zettler
E, Amils R (2010) Phototrophic biolfilms from Rio Tinto, an extreme
acidic environments, the prokaryotic component. In: Seckbach J, Oren
A (eds) Microbial mats. Modern and ancient microorganisms in
stratified systems. Springer, Heidelberg, pp 471–481
Alazard D, Joseph M, Battaglia-Brunet F, Cayol JL, Ollivier B (2010)
Desulfosporosinus acidiphilus sp. nov.: a moderately acidophilic
sulfate-reducing bacterium isolated from acid mining drainage
sediments. New taxa: Firmicutes (Class Clostridia, Order
Clostridiales, Family Peptococcaceae). Extremophiles 14:305–312
Amaral-Zettler LA, Go ´mez F, Zettler E, Keenan BG, Amils R, Sogin
ML (2003) Eukaryotic diversity in Spain’s River of Fire. Nature
417:137
Basen M, Sun J, Adams M (2012) Engineering a hyperthermophilic
archaeon for temperature-dependent product formation. mBio 3:
e00053-12. doi:10.1128/mBio.00053-12
Ben Hania W, Ghodbane R, Postec A, Brochier-Armanet C, Hamdi M,
Fardeau M, Ollivier B (2011) Cultivation of the first mesophilic
representative (“mesotoga”) within the order Thermotogales. Syst
Appl Microbiol 34:581–585
Benz M, Brune A, Schink B (1998) Anaerobic and aerobic oxidation of
ferrous iron at neutral pH by chemoheterotrophic nitrate-reducing
bacteria. Arch Microbiol 169:159–165
Biddle J, Cardman Z, Mendlovitz H, Albert D, Lloyd K, Boetius A,
Teske A (2012) Anaerobic oxidation of methane at different temperature regimes in Guaymas Basin hydrothermal sediments. ISME
J 6:1018–1031
Borin S et al (2008) Sulfur cycling and methanogenesis primarily
drives microbial colonization of the highly sulfidic Urania deep
hypersaline basin in the Mediterranean Sea. Proc Natl Acad Sci
USA 106:9151–9156
Brochier-Armanet C, Boussau B, Gribaldo S, Forterre P (2008)
Mesophilic Crenarchaeota: proposal for a third archaeal phylum,
the Thaumarchaeota. Nat Rev Microbiol 6:245–252
392
J.-L. Cayol et al.